In administering intravenous fluids to a patient, control of flow rate is critical. If the amount of administered fluid delivered in a given period of time varies considerably from the prescribed dosage rate, the results can be of serious consequence and are often fatal. The most common types of adjustable flow control used today is a clamp device which pinches the IV (intravenous) tubing by an adjustable amount. It has been well documented in the medical literature (see "Flow Rate Maintenance and Output Intravenous Fluid Test Sets", Demorouelle et al, American Journal of Hospital Pharmacy, Vol. 32, pages 177-185, February 1975; and "Regulatory Intravenous Fluid Flow: Controller versus Clamps", Ziser et al, American Journal of Hospital Pharmacy, Vol. 36, pages 1090-1094, August 1979) that such clamps are unable to maintain constant flow rates for any reasonable period of time. The main cause of the problem in this regard is "creep", a phenomenon wherein the plastic tubing diameter, when under stress, continues to change. In addition, the settings of these prior art IV clamps are accessible to curious patients and susceptible to variation upon movement by the patient. Thus, while adjustable IV clamps are very inexpensive, they represent a considerable sacrifice when it comes to maintaining accurate dosage rates.
On the other end of the cost spectrum, the prior art contains automatic systems for precisely maintaining selectable IV flow rates. These systems are generally made up of electrical pumps, sensors and controllers and, for most applications, are prohibitively expensive. As a consequence, it is estimated that sales of such systems comprise less than ten percent of the available market, whereas the much less accurate but inexpensive adjustable clamps account for substantially all other sales.
Still another approach to adjustable IV flow control is in the in-line valve characterized by a valving mechanism which is adjustably inserted directly into the flow path of the IV fluid. Examples of in-line IV valves may be found in U.S. Pat. Nos. 3,880,401 (Waltse); 4,079,737 (Miller); 3,877,428 (Seagle et al); and 3,868,973 (Bierman et al). Such in-line valves tend to be more accurate than the adjustable clamp and far less expensive than the automatic control system. However, such in-line valves have not obtained a meaningful share of the relevant market for two primary reasons. First, the in-line valves are sufficiently more complex and expensive to manufacture than the clamp valves as to make comparative cost an important factor to the user. Second, although not subject to the "creep" phenomenon, the in-line valve is still subject to twiddling by a curious patient and to inadvertent setting variation during patient movement or inadvertent manipulation by health care personnel.